Technical Field
The present invention relates to a wafer level assembly of a MEMS sensor device; in particular, the following description will make reference, without thereby this implying any loss in generality, to the assembly of a MEMS sensor device including an acoustic transducer.
Description of the Related Art
Sensor devices are known including micromechanical structures made, at least in part, of semiconductor materials and made using MEMS (Micro-Electro-Mechanical Systems) technology. These sensor devices are used in portable electronic apparatuses, such as, for example, portable computers, laptops or ultrabooks, PDAs, tablets, mobile phones, smartphones, digital audio players, photographic cameras or video cameras, and consoles for videogames, enabling important advantages to be obtained with regard to size occupation, in terms of area and thickness.
A MEMS sensor device generally comprises: a micromechanical detection structure, designed to transduce a mechanical quantity to be detected (for example, acoustic waves, pressure, etc.) into an electrical quantity (for example, a capacitive variation); and an electronic reading circuit, usually provided as ASIC (Application-Specific Integrated Circuit), designed to execute suitable processing operations (amongst which amplification and filtering operations) of the electrical quantity so as to provide an electrical output signal, whether analog (for example, a voltage) or digital (for example a PDM—Pulse Density Modulation—signal). This electrical signal, possibly further processed by an electronic interface circuit, is then made available for an external electronic system, for example, a microprocessor control circuit of the electronic apparatus incorporating the sensor device.
The micromechanical detection structure of a MEMS acoustic transducer, of a capacitive type, generally comprises a mobile electrode, provided as a diaphragm or membrane, facing a substantially fixed electrode. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whilst a central portion thereof is free to move or bend in response to acoustic-pressure waves incident on a surface thereof. The mobile electrode and the fixed electrode provide the plates of a detection capacitor, and bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of this detection capacitor. During operation, the capacitance variation is converted by suitable processing electronics into an electrical signal, which is supplied as output signal of the MEMS acoustic transducer.
A MEMS acoustic transducer of a known type is, for example, described in detail in patent application No. US 2010/0158279 A1 (to which reference is made herein), filed in the name of the present Applicant.
A portion of the micromechanical detection structure of the acoustic transducer is shown by way of example in FIG. 1, being designated as a whole by 1.
The micromechanical detection structure 1 comprises a substrate 2 made of semiconductor material, and a mobile membrane (or diaphragm) 3. The membrane 3 is made of conductive material and faces a fixed electrode or rigid plate 4, generally known as “back plate”, which is rigid, at least if compared with the membrane 3, which is, instead, flexible and undergoes deformation as a function of the incident acoustic-pressure waves.
The membrane 3 is anchored to the substrate 2 by means of membrane anchorages 5, formed by protuberances of the membrane 3, which extend from peripheral regions of the same membrane 3 towards the substrate 2.
For instance, the membrane 3 has, in plan view, i.e., in a horizontal plane of main extension, a generically square shape, and the membrane anchorages 5, which are four in number, are set at the vertices of the square.
The membrane anchorages 5 suspend the membrane 3 over the substrate 2, at a certain distance therefrom; the value of this distance is the result of a compromise between the linearity of response at low frequencies and the noise of the acoustic transducer.
In order to enable relief of residual (tensile and/or compressive) stresses in the membrane 3, for example, stresses deriving from the manufacturing process, through openings 3′ may be formed through the membrane 3, in particular in the proximity of each membrane anchorage 5, in order to “equalize” the static pressure present on the surfaces of the same membrane 3.
The rigid plate 4 is formed by a first plate layer 4a, made of conductive material and set facing the membrane 3, and a second plate layer 4b, made of insulating material.
The first plate layer 4a forms, together with the membrane 3, the detection capacitor of the micromechanical detection structure 1.
In particular, the second plate layer 4b overlies the first plate layer 4a, except for portions in which it extends through the first plate layer 4a so as to form protuberances 6 of the rigid plate 4, which extend towards the underlying membrane 3 and have the function of preventing adhesion of the membrane 3 to the rigid plate 4, as well as of limiting the oscillations of the same membrane 3.
For instance, the thickness of the membrane 3 is comprised in the range 0.3-1.5 μm, for example, it is equal to 0.7 μm, the thickness of the first plate layer 4a is comprised in the range 0.5-2 μm, for example, it is equal to 0.9 μm, and the thickness of the second plate layer 4b is comprised in the range 0.7-2 μm, and, for example, it is equal to 1.2 μm.
The rigid plate 4 moreover has a plurality of holes 7, which extend through the first and second plate layers 4a, 4b, have, for example, a circular cross section, and allow, during the manufacturing steps, removal of the underlying sacrificial layers. Holes 7 are, for example, arranged so as to form a lattice in a horizontal plane, parallel to the substrate. Moreover, in use, holes 7 enable free circulation of air between the rigid plate 4 and the membrane 3, in effect rendering the rigid plate 4 acoustically transparent. Holes 7 hence provide an acoustic port, to enable acoustic-pressure waves to reach and deform the membrane 3.
The rigid plate 4 is anchored to the substrate 2 by means of plate anchorages 8, which are joined to peripheral regions of the same rigid plate 4.
In particular, plate anchorages 8 are formed by vertical pillars (i.e., pillars extending in a direction orthogonal to the horizontal plane and to the substrate 2), made of the same conductive material as the first plate layer 4a and hence forming a single piece with the rigid plate 4; in other words, the first plate layer 4a has prolongations that extend as far as the substrate 2, defining the anchorages of the rigid plate 4.
The membrane 3 is suspended over and directly faces a first cavity 9a, formed within and through the substrate 2, by a through trench formed, such as by etching, starting from a back surface 2b of the substrate 2, which is opposite to a front surface 2a thereof, on which the membrane anchorages 5 rest (the first cavity 9a hence defines a through hole that extends between the front surface 2a and the rear surface 2b of the substrate 2); in particular, the front surface 2a lies in the horizontal plane.
The first cavity 9a is also known as “back chamber”, in the case where the acoustic-pressure waves impinge first upon the rigid plate 4, and then upon the membrane 3. In this case, the front chamber is formed by a second cavity 9b, delimited at the top and at the bottom, respectively, by the first plate layer 4a and the membrane 3.
Alternatively, it is in any case possible for the pressure waves to reach the membrane 3 through the first cavity 9a, which in this case provides an acoustic access port, and, hence, a front chamber.
In greater detail, the membrane 3 has a first main surface 3a and a second main surface 3b, which are opposite to one another and face, respectively, the first and second cavities 9a, 9b, hence being in fluid communication, respectively, with the back chamber and the front chamber of the acoustic transducer.
Moreover, the first cavity 9a is formed by two cavity portions 9a′, 9a″: a first cavity portion 9a′ is arranged at the front surface 2a of the substrate 2 and has a first extension in the horizontal plane; the second cavity portion 9a″ is set at the rear surface 2b of the substrate 2 and has a second extension in the horizontal plane, greater than the first extension.
In a known way, the sensitivity of the acoustic transducer depends upon the mechanical characteristics of the membrane 3, as well as upon the assembly of the membrane 3 and of the rigid plate 4 within a corresponding package, which constitutes the interface of the acoustic transducer with respect to the external environment.
In particular, the performance of the acoustic transducer depends on the volume of the back chamber and the volume of the front chamber. The volume of the front chamber determines the upper resonance frequency of the acoustic transducer, and hence its performance at high frequencies; in general, in fact, the smaller the volume of the front chamber, the higher the upper cut-off frequency of the acoustic transducer. Moreover, a large volume of the back chamber enables improvement of the frequency response and the sensitivity of the same acoustic transducer.
The package of the acoustic transducer has to be configured to house not only the micromechanical detection structure 1, but also the reading electronics associated thereto, generally provided as an ASIC, electrically coupled to the micromechanical detection structure 1. At the design stage, the fact is also to be considered that acoustic transducers typically operate in unfavorable working environments, for example, ones subject to high RF radiation and electromagnetic disturbance (when integrated in mobile phones or similar wireless communication devices).
Several constraints are thus imposed on the assembly of the MEMS acoustic transducer and the corresponding package, which render design thereof particularly problematical, where compact dimensions are preferred.
An assembly arrangement that has been proposed envisages providing two distinct dice made of semiconductor material, a first die for the micromechanical detection structure and a second die for the reading circuitry.
In a solution of this type, illustrated schematically in FIG. 2 (and described, for example, in U.S. Pat. No. 6,781,231), a first die 10, integrating the micromechanical detection structure 1 (illustrated schematically herein), and a second die 11, integrating an ASIC 11′ of the reading electronics, are coupled side by side on a supporting layer 12 of a corresponding package 14. Electrical connections 15 between the first and second dice 11, 12 are provided by means of electrical wires with the wire-bonding technique, whilst appropriate metallization layers and vias (not illustrated in detail) are provided in the supporting layer 12 for routing the electrical signals towards the outside of the package 14.
A cover 16 of the package 14 is moreover coupled to the supporting layer 12, enclosing the first and second dice 11, 12; this cover 16 may be made of metal or pre-molded plastic with internal metallization layers such as to prevent disturbance of external electromagnetic signals (by providing a sort of Faraday cage).
The cover 16 moreover has an opening 18 to enable entry of acoustic-pressure waves. Advantageously, a screen for the incident light (not illustrated), or a filter (not illustrated either) may be coupled to opening 18, to prevent penetration into the cover 16 of particles of dust or other material.
Pads (not shown) are provided at the underside of the supporting layer 12 for soldering and electrical connection to an external printed circuit.
This assembly arrangement is not, however, free from drawbacks, amongst which the fact of preferring large dimensions for accommodating the two dice of the acoustic transducer side by side and for providing the corresponding package.
Moreover, this solution does not offer the designer much freedom (as, instead, would be desirable) in the sizing of the chambers of the acoustic transducer, for the determination of its electrical characteristics.
Various assembling and packaging solutions for MEMS acoustic transducers have consequently been proposed, amongst which, for example, those described in U.S. Pat. No. 6,088,463, US 2007/0189568, WO 2007/112743, EP 2 252 077, EP 2 517 480.
However, also these solutions are not optimized as regards the dimensions, assembly costs, and electrical characteristics of the sensors.
The need is consequently felt in the field to provide an appropriate assembly of a MEMS sensor device, which can provide low manufacturing costs, high performance and reliability, and contained dimensions to be met.